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What port setup are you using with the sigma 17-70? i was looking at getting one, currently i have a 4" dome and AF/MF macro port

with a minimum focusing of 14 inches (36cm) you will likely be required to use a close up lens of at least +2 in order to focus on the virtual images that will be +/- less at 10.5 inches (36cm) in front of the dome, that will further reduce the field of view of your lenses (there are formula fo calculating this, but I am both too vintage and too lazy to rememebr those by hearth ) if you have a larger dome you might be able to dispense with the +2 close up lens (diopter).

Incidentally, the close focusing is one of the main reason the Sigma 17-70mm is more popular, it has a really good minimum focusing distance (on mine its 20 cm / 7.9 inches, the newer version has 22 cm / 8.7 in for a minimum distance).